Why is Rowan Williams suspicious of Freemasonry? For the same reason, surely, that almost everyone else is: it is a secretive society with links to mystical gobbledegook that may or may not have a whiff of Satanism. A Christian leader should be particularly careful not to be associated with such stuff, for he wants the Christian faith to seem open, accessible, reasonable – utterly distinct from such cloak-and-dagger Harry Potter stuff.

Well, yes, but there is actually another dimension to Williams' aversion, which is somewhat counter-intuitive. Freemasonry may have links with ancient magic, but it also has links with modern reason. I think that this is what Williams really dislikes about it. Not the funny handshakes and creepy initiation ceremony, but the implicit claim that the rationalist God of the Enlightenment is an improvement on the limited Christian one.

In the 18th century, Freemasonry spread among middle-class men who felt that religion should modernise; it should be about rational moral progress, and it should unite people rather than keep old divisions alive. And it especially appealed to those who felt that rationalism was in danger of looking dull, mechanical, soulless. So it needed spicing up with an aura of mystery, and some cod ancient history, and some ritual. It often had a strong anti-Catholic bent; it was a secularisation of the liberal Protestant hatred of Church power.

Williams' suspicion of this tradition goes to the very heart of his theology. It is of a piece with his suspicion of the Enlightenment, and of "liberalism". The essence of liberalism, in this view, is its claim that the rational good of humanity has superseded any particular religion, including Christianity. Every faith must be judged by universal moral values. And this ideology gives a huge implicit blessing to capitalism, as the natural progressive state of things.

Above all, it confers a quasi-religious aura on the liberal state. The idea of America as "the world's best hope", in Jefferson's phrase, is a blasphemy: the Christian church is.

Of course he is not alone in this analysis; it is the essence of postmodern theology. It draws on the great Protestant thinker Karl Barth, who denounced liberal Protestantism as a sell-out to deism, and on Catholic thought, and on various non-religious thinkers, dismissive of arrogant rationality. But Williams has been one of its chief exponents in recent decades; he helped to inspire the Radical Orthodoxy movement within academic theology, which is now the dominant school of thought.

I consider this anti-liberal form of theology to be justified to a large extent, yet also flawed. It is quite right that theology was swamped by deist assumptions during modernity, that it forgot the primacy of its own language, and of its ritual practices. But its rejection of liberalism is over-hasty, clumsy. It fails to see that there was – and still is – something good, even sacredly good, in liberalism. This can be summed up in two simple words: religious liberty. The postmodern Catholic and Anglo-Catholic critics of "liberalism" should acknowledge that the ideal of religious liberty, which actually arose on Christian rather than Enlightenment soil, cannot be so easily dismissed. Quite apart from its secular benefits, it has helped to purify Christianity. Yes, there is a danger of absolutising the liberal state. But there is a contrary danger too, of undervaluing it.